1. Field of the Invention
The present invention relates to a method for forming a device substrate and more particularly to a method for forming a device substrate with a metal back-gate and the structure formed thereby.
2. Description of the Related Art
Laminating a thin metal layer (e.g., tungsten (W)) between silicon-based materials such as a gate oxide and a buried oxide is a key step to making substrates for double-gate devices with a metal back-gate. Because of its flexibility with all kinds of materials, even if the materials are polycrystalline, amorphous, or single crystalline but with very different lattice mismatch, wafer bonding is a promising approach to make this multilayer structure.
However, for the bonding process, after the room-temperature joining step, a thermal treatment at 1100° C. is commonly used to enhance the bonding strength. Due to the weak adhesion at the interface between metals and silicon oxide as a result of their chemical and physical incompatibility such as thermal mismatch, the stacked layers are very likely to disintegrate during the high temperature bonding anneal in the form of delamination at weak interfaces. This problem has been observed experimentally and this conventional process is schematically shown in FIG. 1. In the case of tungsten bonding with silicon dioxide, tungsten reacts with oxygen from silicon dioxide to form tungsten oxide (WO3), which its volume increases dramatically (3×) and causes delamination, during high temperature bonding anneal.